This invention relates to a method of controlling an oxygen generating device and to an oxygen generating device.
In order for passengers or aircrew in an aircraft to breath when exposed to ambient atmospheric pressure at elevated altitudes, it is necessary to provide a supply of breathing gas enriched with oxygen.
One means of achieving this is to carry within the airframe a supply of compressed oxygen gas, but particularly in a small aircraft, where space is at a premium, and/or in an aircraft where the added weight of the gas bottle containing the compressed oxygen gas is significant, this is not acceptable.
To reduce weight and space requirements another means is to carry within the airframe liquid oxygen. Liquid oxygen systems give rise to space and weight penalties and also a requirement for liquid oxygen to be available for replenishment of the liquid oxygen supply at a ground station.
More recently oxygen-enriched gas has been produced on-board of the aircraft by a so-called on-board oxygen generating system (OBOGS) based on pressure swing technology using a zeolite molecular sieve material to separate oxygen from air. This requires at least two zeolite beds which have to be sequentially cycled through on-stream/generating and off-stream/purge cycles. A limitation of such systems is that theoretically the maximum oxygen concentration obtainable in the product gas is 95% unless additional means are provided for the removal of argon and other trace gases from the supply air which is normally bleed air from a compressor stage of an engine powering the aircraft.
Increasing attention is now being given to ceramic membrane technology in provision of a system which will generate substantially 100% oxygen product gas or highly oxygen-enriched product gas of breathable quality for use in aerospace and other breathing applications. Such gas will hereinafter be referred to as being "oxygen rich", and the residual gas, will be referred to as being "oxygen depleted".
Certain ceramic materials (for example Yttria doped Zirconia or Gaddia doped Ceria), which are so-called ionic conductors of oxygen, become electrically conductive at elevated temperatures due to the mobility of oxygen ions within the crystal lattice. Since these materials are only conductive to oxygen ions in the presence of an electrical current, an external electric circuit is needed. It is necessary to control the electrical current supply in order to regulate the production of oxygen required.
Such an oxygen generation device comprises a membrane of such material to one side of which is supplied ambient air. Oxygen diffuses through the membrane by ionic transport and is recoverable from a second, other, side of the membrane for use. Oxygen production rate is dependent on the electrical current supply to the membrane.